Field of the Invention
The invention disclosed in this specification relates to a motor control device.
Description of Related Art
In order to drive a brushless DC motor without a sensor, it is necessary, for example, to measure counter electromotive forces of phases of the motor, to perform zero-crossing detection, and to obtain a rotational position of a rotor.
Note that as an example of a conventional technique related to above description, there is JP-A-2014-87210 filed by the applicant of this application.
FIG. 19 is a block diagram illustrating a conventional zero-crossing detection method (first conventional example). In the zero-crossing detection method of this diagram, each phase of the motor is provided with an AD converter, and hence counter electromotive forces of phases can be simultaneously AD converted and used for zero-crossing detection process by a microcomputer. However, this conventional example has a problem that multiple AD converters are needed.
FIG. 20 is a block diagram illustrating a conventional zero-crossing detection method (second conventional example). In the zero-crossing detection method of this diagram, each phase of the motor is provided with a sample and hold unit. Therefore, counter electromotive forces of phases can be simultaneously sampled, and their hold values can be sequentially AD converted by a single AD converter. Thus, the number of AD converters in this conventional example can be smaller than that in the first conventional example described above. However, the sample and hold units are additionally needed in this conventional example, and hence further improvement is required in view of circuit scale.
In addition, when motor rotational speed is PWM controlled, noise or rise delay of an induced voltage (including a variation among phases) occurs at or near an on-off switch timing when a PWM on period and a PWM off period are switched. However, in the conventional zero-crossing detection method, sampling timings of the AD converter are always set to the PWM on period regardless of a PWM duty. Therefore, in low-duty drive (i.e. low-speed rotation) in which the PWM on period becomes short, even if each phase of the motor is provided with the AD converter or the sample and hold unit, the sampling timing of the AD converter becomes close to the on-off switch timing, and hence the zero-crossing detection may not be correctly performed.